A Combined Experimental and Theoretical Study of Uranium Polyhydrides with New Evidence for the Large Complex UH₄(H₂)₆

Abstract

Several monouranium and diuranium polyhydride molecules were investigated using quantum chemical methods. The infrared spectra of uranium and hydrogen reaction products in condensed neon and pure hydrogen were measured and compared with previous argon matrix frequencies. The calculated molecular structures and vibrational frequencies were used to identify the species present in the matrix. Major new absorptions were observed and compared with the previous argon matrix study. Spectroscopic evidence was obtained for the novel complex, UH 4 (H 2 ) 6 , which has potential interest as a metal hydride with a large number of hydrogen atoms bound to uranium. Our calculations show that the series of complexes UH 4 (H 2 ) 1,2,4,6 are stable.

Infrared Spectra of ThH2, ThH4, and the Hydride Bridging ThH4(H2)x (x = 1−4) Complexes in Solid Neon and Hydrogen

“…Our calculation predicts this splitting to be 16 cm -1 , and we observe it to be 15 cm -1 . Absorptions for ThH 4 (H 2 ) 2 and ThH 4 (H 2 ) 3 were observed at 1404 and 1388 cm -1 , respectively, in the argon matrix, which shift to 1006 and 994 cm -1 with deuterium. However, these bands are weak in solid neon and hydrogen.…”

“…In contrast, UH is produced in higher yield, and its rhombic dimer is observed. 3,28 CASPT2 calculations indicate that HThThH has a rhombic structure and a 1 A 1g ground state, while DFT predicts a linear 3 A g ground state. 29 Bonding.…”

“…1 Recently, we reported the evidence for six hydrogen molecules bound to the uranium tetrahydride center. 3 The 5f orbitals must play a very important role in bonding the dihydrogen ligands. A different complex ThH 4 (H 2 ) 4 and structure are found for the thorium case, in which H 2 ligands are bridging the hydride centers suggesting a different bonding mechanism within the Th species.…”

“…For example, uranium atoms react with H 2 to give UH 4 , which in solid hydrogen ultimately forms the supercomplex UH 4 (H 2 ) 6 . 3 This molecule has potential interest as a metal hydride with a large number of hydrogen atoms bound to uranium. In contrast, Mo and W also form tetrahydride molecules, but calculations suggest that these coordinate only four dihydrogen ligands to give MH 4 (H 2 ) 4 complexes.…”

“…Owing to the unique chemistry of thorium, the structure of this complex is computed to be different from the uranium and tungsten analogues. 3,6 …”